Acoustic device



Dec. 11, 1934. I BALLANTI-NE' 1,983,833

I ACOUSTIC DEVICE I Filed Dec. 8, 1930 s Sheets-Sheet 1 Response Frequen y 0 E/ecfr/ca/ gnaw Jot 4 f r'e?uenc (Cycle; Ar a) Dec. 11, 1934. s. B ALLANTINE ,983,8

ACOUSTIC DEVICE I Filed Dec. 8, 1930 :5 Sheets-Sheet 2 gwuentot Dec. 11, 1934.

s. BALLANTINE 1,983,833

ACOUSTIC DEVICE Filed Dec. 8, 1930 3 Sheets-Sheet 5 Patented Dec. 11, 1934 UNITED STATES PATENT OFFICE ACOUSTIC DEVICE Delaware Application December 8, 1930, Serial No. 500,916

4 Claims.

This invention relates to acoustic devices of the type including an exposed diaphragm which is adapted to be vibrated by pressure waves to produce an electrical wave, or to be vibrated by electrical energy to produce a sound wave.

The invention will be described as applied to a condenser microphone of the type extensively employed for precision recording and measurement of sound. Examples of the construction of such microphones will be found in the United States patents to E. C. Wente, No. 1,333,744, and I. B. Crandall, No. 1,456,538, and it is to be'noted that the constructions illustrated therein are typical of the previously known condenser microphones in that they provide a shallow recess in front of the diaphragm, the recess being formed by the ring used for stretching the diaphragm.

Such diaphragms are ordinarily calibrated by applying to the membrane an alternating pressure whose frequency can be varied over the audio range and whose amplitude is accurately known. The electrical output resulting from such application of pressures of known frequencies and amplitudes was noted, the data thus obtained being thereafter used in interpreting the measurements of electrical output obtained when the microphone was placed in the sound field under investigation.

I have discovered that such a calibration does not accurately represent the performance of the microphone in a free sound field. This is due to the fact that the actual pressure at the face of the diaphragm is greater than the pressure in the undisturbed sound wave.

In the first place, the pressure at the diaphragm is increased by reflection by a factor which varies with the frequency from unity at low frequencies to two at high frequencies. Secondly, the pressure at the diaphragm is still further increased by resonance in the cavity in front of the diaphragm.

So far as I am aware, these effects were not previously. known, or, if known, no adequate method was available for determining the pressure in the undisturbed sound field from the observed pressure at the diaphragm, i. e., from the observed electrical output.

An object of the invention is to provide a microphone having such characteristics that the ratio of the pressure on the diaphragm to that in the undisturbed sound field may be accurately predetermined. A further object is to provide a microphone which, for. all audio frequencies, exhibits substantially the same electrical output for a givenunit' pressure in the undisturbed sound for spherical mountings of field. More particularly, an object is to provide a microphone in which the cavity resonance and/or reflection effects may be adjusted to provide a frequency variant increase of pressure on the face of the diaphragm to compensate the decrease of pressure, with frequency, which occurs in ordinary damped membrane systems. A further object is to provide an acoustic device of the exposed diaphragm type and in which the effects of cavity resonance are completely eliminated.

These and. other objects will be apparent from the following specification, when taken with the accompanying drawings, in which:

Fig. 1 is a curve sheet showing the variation, with frequency, of the increase of response of a condenser microphone due to cavity resonance and reflection,

Fig. 2 is a horizontal central section through one embodiment of the invention,

Fig. 3 is a perspective view of the same,

Fig. 4 is a curve sheet showing, for a uniform pressure in the undisturbed sound field, the frequency variation of the response for an ordinary air damped diaphragm and the compensation which may beeffected by an appropriate evaluation of cavity resonance and/ or reflection effects, and

Fig. 5 is a fragmentary sectional view of a microphone for which the cavity resonance may be varied.

The magnitude of the error arising from the usual assumption that the pressure at the diaphragm is that which exists in the undisturbed sound field is shown, for one typical case, by the curves of Fig. l. The cavity at the base of which the diaphragm was mounted was approximately 4 cm. in diameter and 1 cm. deep and, to permit accurate calculation of the pressure increase due to reflection, the microphone mounting was of spherical form. A discussion of pressure increase due to reflection and the corrections which are to be applied to the observed pressure to obtain the actual pressure in the undisturbed sound field various diameters will be found in my article Effect of Diffraction Around the Microphone in Sound Measurements,

.which appeared in Physical Review, December, 1928, pages 988-992.

In Fig. 1, the curve 1 shows the ratio, as determined experimentally by means of the Rayleigh disk, of the pressure at the diaphragm to the pressure in the undisturbed sound wave.- Curve 2 represents the increase in pressure due to reflection from a spherical microphone housing, as calculated in accordance with the discussion given in the Physical Review" paper, and curve 3 represents the calculated effect of resonance in the shallow cavity in front of the microphone membrane. It will be seen that curve 1, representing the operation of both effects, is roughly equal to the product of the individual effects represented by curves 2 and 3.

It will be observed that the effects are very large ones and cause a considerable departure of the overall response curve from the response which would be predicted from the usual calibration in terms of pressure at the face of the diaphragm. At 3500 cycles, for example, the error committed would amount to over 300%.

A microphone mounting which eliminates the effects of cavity resonance is illustrated in Figs. 2 and 3. the back plate 5 to form the condenser unit may be formed of duralumin, aluminum, magnesium or some other light alloy, and constitutes, in effect, a portion of the exterior surface of the microphone housing. Topermit accurate computation of the pressure increase due to reflection, the housing is preferably of spherical form and may, as illustrated, be constituted by I shaped plates 6 having meeting edges which are so overlapped as to avoid the formation of projecting ridges. The diaphragm 4 is held in a clamping head formed by outer annular member? and an inner ring 8, which members may be, and preferably are, formed of steel. The diaphragm 4 is placed under tension by a straining ring 9 which slides within the inner ring 8 and is forcedoutwardly by the follower which is threadedto ring 4.

The straining ring 9 has an internal shoulder against which is seated the outer end of a cylinder 11, the ring 12 being threaded upon the interior wall of the follower 10 to maintain the cylinder 11 in place. The back plate 5 is carried by the outer flanged end of cylinder 11, being held thereon by the nut 13, and mica washers 14 are located at the inner and outer faces of the flanged end of the cylinder 11 to insulate the back plate 2 from the remainder of the microphone and its housing. Equalization of pressure,

in the space back of diaphragm 4, in response to barometric variations may be effected by the natural leakage around the parts, increased if desired by shallow grooves in tf 2 flanged end of cylinder 11, or by a special vent 15 which is long and of capillary diameter thus avoiding inadvertent fluctuations of pressure in the audio range which might arise from resonance between the inertance of the plug of air in a relatively large vent and the compressibility of the air around the back plate.

To avoid long leads, it is customary to mount the first stage of the. amplifier close to the microphone. The spherical housing is" particularly conve 'ent in this respect since it provides ample space or the amplifier stage, indicated diagrammatically by the rectangular outline Amplifier Stage in Fig. 2, within the housing. Suitable leads 16 extend from the clamp ring 8 and the back plate 5, respectively, to the amplifier terminals 17. In one microphone, I have employed sheet lead for the housing with good results. The lead acts as an electrical shield and also prevents sound waves from acting upon the back of the microphone orthe amplifier.

An arrangement such as shown in Figs. 2 and 3 may be used as a standard for the calibration of other microphones which may or may not be 75 of spherical shape. The absence of cavity reso- The diaphragm 4 which cooperates with nanceand the possibility of accurately computing the effect of reflection results in a definite ratio between observed pressure at the diaphragm and the pressure in the undisturbed sound field.

To reduce the effect of diaphragm resonance upon the electrical output, it is usual to limit the air space back of the diaphragm to secure an air damping action. As shown by curve 20 of Fig. 4, the air damping and diaphragm resonance will, in general, be such as to result in a falling off of the electrical output with increasing frequency. The curves of Fig. 1 show that, for constant pressure in an undisturbed sound field, the

-actual pressure at the diaphragm increases with frequency due to reflection and cavity resonance. Curve 21 of Fig. 4 represents the calculated increase of diaphragm pressure for a spherical microphone housing of 5 inches in diameter, and curve 22 shows the variation of the overall electrical response with frequency when a microphone having such properties as will give rise to response curve 20 is mounted in a spherical housing exhibiting the characteristics shown by response curve 21. It will be noted that this housing has a radius of the same order of magnitude as the wave length of sound in air at the frequencies where the increase of pressure is desired. This particular combination effects a substantial compensation between the pressure decrease due to air damping and the pressure rise due to reflection, thus giving a response curve that is substantially of frequencies.

The same compensating effect may be derived from cavity resonance by proper choice of the dimensions of the cavity and air damping. Due pression for determining the frequency variation of cavity resonance and reflection effects, I have found the best design procedure in compensating for air damping to be an experimental one. The dimensions as given above and the curves of Fig. 1 will afford one skilled in the art a clear idea of the compensation which may be effected when the air damping, the size of the microphone housing and/or the cavity dimensions are properly evaluated. The experimental procedure can readily be'carried out by one skilled in the art, once the existence of the opposing effects and the understood.

-0ne convenient method is to adopt definite fixed dimensions for the shell. in which aparticular microphone is to be housed, and then adjust the resonance of the cavity by inserting tubes of various length in front of the diaphragm. As shown in Fig. 5, thediaphragm is secured between the outer ring 25 and inner ring 26 of the clamping head, and is tensioned by the stretching ring 2'7. Thefollower 28 the stretching ring in place, and a tube 29 is slidable within the bore provided by ring 27 and follower 28 to control the effective depth of the ca'vity. Alternatively, tubes of different length may be substituted for the tube 29,'or tubes of different wall thickness may be substituted tmchange the eifective diameter ofthe cavity.

While the invention has been described in connection with the condenser microphone, it is apparent that the same problems arise in other pick-up instruments of the exposed diaphragm type, such as for example, the double carbon button microphone used extensively in radio broad-I casting and public address systems. It will also principle of compensation are threads into ring 25 to hold fiat over a wide range to the complexity of a mathematical exbe apparent that, by reciprocity, certain of the features described in connection with microphones apply equally well to sound reproducers of the exposed diaphragm type.

I claim:

1. In a microphone, the combination with a pick-up device of the type including an exposed diaphragm and an air damping chamber at the rear of said diaphragm, of a spherical housing for said pick-up device having a radius of the order of magnitude of the wave length of sound waves in air at the highest sound frequencies to which the microphone is to respond, whereby the frequency variation of response arising from air damping is reduced by pressure increase due to reflection by said housing.

2. A microphone comprising an outer clamping ring, an inner clamping ring, a diaphragm having its periphery held between said rings, a straining ring back of said diaphragm, and a spherical housing supporting said outer clamping ring, the outer face of the said 0 ter clamping ring and the exposed portion of said diaphragm conforming substantially to the spherical shape of said housing.

3. A microphone comprising an acoustically responsive diaphragm mounted substantially in the plane of the surface of a housing, the said housing having a volume substantially that of a sphere having a radius of the same order of magnitude as the wave length of sound in air at frequencies where pressure increase upon the said diaphragm by reflection fromthe surface of the said housing is desired.

4. A microphone comprising an acoustically responsive diaphragm mounted in the plane of a surface of a housing, the said housing producing pressure increase upon the diaphragm by reflection from its surface at certain frequencies, and the said housing having a volume substantially that of a sphere having a radius of the same order of magnitude as the wave length of sound in air at the frequencies where pressure increase upon the said diaphragm by reflection from the surface of the said housing is desired. 1 v

STUART BALLANTINE. 

